Metals - Extraction of Aluminum (Bauxite)

Aluminum is extracted from its primary ore, bauxite, which is purified to form alumina ($Al_2O_3$).

Because aluminum is more reactive than carbon, it cannot be reduced by carbon heating; instead, electrolysis of the molten oxide is required.

The melting point of pure $Al_2O_3$ is very high (over $2000^{\circ}C$). It is dissolved in molten cryolite ($Na_3AlF_6$) to lower the melting point to approximately $950^{\circ}C$, which saves energy and costs.

Cryolite also improves the electrical conductivity of the electrolyte.

The electrolysis cell uses carbon (graphite) for both the anode (positive electrode) and the cathode (negative electrode).

At the cathode, aluminum ions ($Al^{3+}$) undergo reduction to form liquid aluminum metal: $Al^{3+} + 3e^{-} \rightarrow Al(l)$.

At the anode, oxide ions ($O^{2-}$) undergo oxidation to form oxygen gas: $2O^{2-} \rightarrow O_2(g) + 4e^{-}$.

The oxygen gas produced at the anode reacts with the graphite electrodes at high temperatures to produce carbon dioxide gas ($C + O_2 \rightarrow CO_2$), meaning the anodes wear away and must be replaced regularly.